Decision for issue #780.
Page reordering and same-Page Block movement remain two route-owned behaviors. They use dnd-kit directly inside the Rulebook route composition. No shared sortable-hierarchy component is earned.
NestedTabs, DocumentEditorLayout, and the editor state manager do not own drag behavior. NestedTabs.Group gains only a polymorphic root so route composition can register an empty Block region as a drop target without turning the Group into a tab.